The present invention relates to the growth of silicon carbide epitaxial layers. As a semiconductor material, silicon carbide is particularly superior for high power, high frequency, and high temperature electronic devices. Silicon carbide has an extremely high thermal conductivity, and can withstand both high electric fields and high current densities before breakdown. Silicon carbide's wide band gap results in low leakage currents even at high temperatures. For these and other reasons, silicon carbide is a quite desirable semiconductor material for power devices; i.e., those designed to operate at relatively high voltages.
Silicon carbide is, however, a difficult material to work with. Growth processes must be carried out at relatively high temperatures, above at least about 1500.degree. C. for epitaxial growth and approximately 2200.degree. C. for sublimation growth. Additionally, silicon carbide can form over 150 polytypes, many of which are separated by small thermodynamic differences. As a result, single crystal growth of silicon carbide, either by epitaxial layer or bulk crystal, is a challenging process. Finally, silicon carbide's extreme hardness (it is most often industrially used as an abrasive material) contributes to the difficulty in handling it and forming it into appropriate semiconductor devices.
Nevertheless, over the last decade much progress has been made in growth techniques for silicon carbide and are reflected, for example, in U.S. Pat. Nos. 4,912,063; 4,912,064; Re. Pat. No. 34,861; U.S. Pat. Nos. 4,981,551; 5,200,022; and 5,459,107; all of which are either assigned, or exclusively licensed, to the assignee of the present invention. These and other patents that are commonly assigned with the present invention have sparked worldwide interest in growth techniques for silicon carbide and thereafter the production of appropriate semiconductor devices from silicon carbide.
One particular growth technique is referred to as "chemical vapor deposition" or "CVD." In this technique, source gases (such as silane SiH.sub.4 and propane C.sub.3 H.sub.8 for silicon carbide) are introduced into a heated reaction chamber that also includes a substrate surface upon which the source gases react to form the epitaxial layer. In order to help control the rate of the growth reaction, the source gases are typically introduced with a carrier gas, with the carrier gas forming the largest volume of the gas flow.
Chemical vapor deposition (CVD) growth processes for silicon carbide have been refined in terms of temperature profiles, gas velocities, gas concentrations, chemistry, and pressure. The selection of conditions used to produce particular epilayers is often a compromise among factors such as desired growth rate, reaction temperature, cycle time, gas volume, equipment cost, doping uniformity, and layer thicknesses.
In particular, and other factors being equal, uniform layer thicknesses tend to provide more consistent performance in semiconductor devices that are subsequently produced from the epitaxial layers. Alternatively, less uniform layers tend to degrade device performance, or even render the layers unsuitable for device manufacture.
In typical CVD processes, however, a phenomenon known as "depletion" occurs and is described as the loss of source gas concentration as the source and carrier gases pass through the reaction vessel. More particularly, in typical CVD systems, the source and carrier gases flow parallel to the substrate and the epitaxial growth surface. Because the source gases react to form the epitaxial layers, their concentration tends to be highest at the gas entry or "upstream" end of the reactor and lowest at the downstream end. In turn, because the concentration of source gases decreases during the travel of the source gases through the reactor, epitaxial layers tend to result that are thicker at the upstream end and thinner at the downstream end. As noted above, this lack of uniformity can be disadvantageous in many circumstances, and is particularly troublesome when thicker epitaxial layers are desired or necessary for certain devices or device structures.
In growth techniques for other semiconductor materials (such as silicon), the problem can be addressed by fairly straightforward techniques such as rotating the substrate (typically a wafer) upon which the epitaxial layer is being grown. Such techniques become much more complex and difficult, however, when carried out at the much higher temperatures required to grow epitaxial layers of silicon carbide. Typically, the susceptors used for silicon carbide growth processes must be formed from highly purified graphite with a high purity coating of silicon carbide. When moving parts are formed from such materials, they tend to be rather complex and prone to generate dust because of the abrasive characteristics of the silicon carbide. Thus, such mechanical and motion-related solutions to the depletion problem are generally unsatisfactory for silicon carbide because of the mechanical difficulties encountered and the impurities that must otherwise be controlled. Accordingly, a need exists for chemical vapor deposition techniques for the epitaxial growth of silicon carbide that produce more uniform epitaxial layers, and yet do so without introducing additional impurities or mechanical complexity to the process.